Thanks in advance for any assistance.

1. Changing the camera angle. I read that this might be possible, pending OpenGL hardware acceleration which at the time (2012) wasn't implemented.
See here: viewtopic.php?t=108741
and here: https://docs.kde.org/stable5/en/kdeedu/marble/faq.html

So what's the status of hardware acceleration, and camera angle adjustments?

2. My previous question was partly because I want to include terrain elevation data in my maps, even if the terrain cannot be rendered/displayed in 3D/as a relief map. Does Marble currently support elevation data? If I am constructing a Marble Map, how do I include GeoTiffs with elevation data (my current data set is 30m resolution)?

3. I'm trying to find the right combination of tutorials/forum posts to achieve the following:
---my maps will be stored locally on the devices running them, rather than pulled from a map server
---my maps are not global. Each Marble Map that I'll build covers a small/medium-sized country, for example Israel or Taiwan
---my maps will combine different raster maps for different zoom levels. Ideally, I'd like to also be able to toggle certain map layers on/off. Should I be using KML Overlays instead?

This seems like I need a combination of ( viewtopic.php?f=217&t=138619 ) and ( viewtopic.php?t=108916 ) and maybe ( https://techbase.kde.org/Marble/HistoricalMaps )....

I've kinda settled on Marble because it's Qt5-based and open-source. All of my other apps are also Qt5/C++ so this streamlines the languages I need to employ. If I can't get Marble to do what I need, I'll have to fall back to upgrading Earth3DLib ( http://www.earth3d.org/docs/earth3d_lib.html ) to Qt5.9 just to render the globe the way I want, and still have to add all the navigational functionality that already exists in Marble anyway.

Sorry for the late reply. Did you succeed?

---my maps will be stored locally on the devices running them, rather than pulled from a map server

That should be fine. You can store all the data locally at ~/.local/share/marble/maps/earth (on Linux)

---my maps are not global. Each Marble Map that I'll build covers a small/medium-sized country, for example Israel or Taiwan

The Israel Hike & Bike map does something similar. See https://marble.kde.org/maps-4.5.php and check the content of the zip file.

---my maps will combine different raster maps for different zoom levels. Ideally, I'd like to also be able to toggle certain map layers on/off. Should I be using KML Overlays instead?

No. I'd rather suggest to define the different layers in the DGML file as done inside the Israel Hike & Bike map.